Yeah, if your late into flower and you have them your options depend on how severe it is. If it is very severe... why the heck weren't you paying more attention?
Late in flowering you can either harvest earlier, or remove the most affected leaves and wait it out. Again, depends how bad... webbing is a bad indication.
Sometimes if you weren't being completely vigilant (rotating, inspecting plants regularly) but were taking some kind of half-assed precaution (which I've done myself), you can still get them on isolated plants or areas. The bigger the plants the more difficult it can be to control. If you have it on isolated plants, take care of them and clean up the best you can after.
In early flower or veg try insecticidal soap, Spinosad, or Azamax/Neem, Organocide (sesame oils) and if you must pyrethrin (indoors) which kills on contact but still breaks down relatively quickly. You can use ladybugs and Spinosad. Ladybugs have no problem hanging out in a grow tent, the more you get the better (they are cheap). SM predator mites are a tad expensive. Whatever you use must be applied every 5-7 days. Wait a couple days before treating again, you want to wait for any surviving eggs to hatch. Some treatments don't generally kill the eggs.
Cooler temps will slow them down, as will high humidity (obv. not for bloom). However, many spider mite variants are capable of going into diapause (overwintering), i.e. they are able to 're-emerge' weeks after you thought they were gone indoors.
hate Spidermites